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Krakow Jan. 2005. R. Yoshida (Argonne). Pentaquark searches at ZEUS. Θ  K S p : Many fixed target experiments report Θ +  K + n (and K S p) at ~ 1530 MeV Ξ 3/2  Ξπ : NA49 reports exotic Ξ 3/2 at ~1860 MeV Θ C  D*p : H1 reports Θ C  D*p at 3099 MeV. Θ  K S p.

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Pentaquark searches at zeus

Krakow Jan. 2005

R. Yoshida (Argonne)

Pentaquark searches at ZEUS

  • Θ KSp : Many fixed target experiments report Θ+ K+n (and KSp) at ~ 1530 MeV

  • Ξ3/2 Ξπ : NA49 reports exotic Ξ3/2 at ~1860 MeV

  • ΘC  D*p : H1 reports ΘC  D*p at 3099 MeV

K s p
Θ KSp

  • Θ+ K+n is a manifestly exotic state (uudds). Observed mass ~1530 MeV.

  • Θ KSp is not manifestly exotic. Can be interpreted as a Σ* (uudds).

  • KSp spectrum has many reported “Σ bumps” (observed by experiments at low significance). 1480 MeV (near threshold), 1560 MeV, 1580 MeV,…

  • A “window” exists 15001550 MeV where no states are reported.

  • A narrow state at ~1530 MeV in KSP would be consistent with the exotic Θ+.

K s p1
Θ KSp

The search is made in Deep Inelastic Scattering

sample: ~121 pb-1, the entire HERA I sample.

electron (triggers event without bias)



Initial baryon




K s p2
Θ KSp

870k Ks reconstructed

(anti)Proton candidates

from dE/dx in the

central tracking

chamber (CTD).

K s p3
Θ KSp


Looking for known states

using same sample and




No Θ++

K s p4

MC studies:

(produce Θ in

the same way

as baryons in


Θ KSp

Spectrum in bins of Q2.

At Q2 ≈10GeV2 or larger,



As Q2  0



K s p5
Θ KSp




M = 1521.5 ± 1.5 MeV

Γ = 8 ± 4 MeV




Monte Carlo normalized


Monte Carlo has no

knowledge of “Σ bumps”

Particle and antiparticle

(Results same for Q2 > 15-30 GeV2)

K s p6
Θ KSp

Determine cross-section using MC (Σ+ forced to be Θ+)

K s p7
Θ KSp

Cross-section ratio to the Λ (1116) determined using Λpπ mode.

Λ(1520)  K+p has very different kinematic selections due to the use of dE/dx for both

K and p.


K s p8
Θ KSp

  • A state at 1521.5 ± 1.5 MeV and width Γ = 8 ± 4 MeV observed in 121 pb-1 of DIS sample in the Ksp spectrum at Q2>20 GeV2.

  • The significance is 3.9-4.6σ (exact number cannot be determined due to the unknown background shape.)

  • Interpretation:

    • Same state as Θ+ pentaquark observed elsewhere. Then:

      • First observation in fragmentation

      • First observation of the anti-pentaquark

    • A so-far-unobserved Σ*.

    • Structure due to some interference. (Reflections from known states were checked and ruled out)

Ξ3/2 Ξπ

NA49 has reported the observation of doubly charged

decuplet partner of the Θ, the Ξ3/2 as well as the Ξ3/2 .


ZEUS has searched for this state in the same 121 pb-1 DIS sample as for the Θ search.

First reconstruct

Λ using dE/dx to ID


Ξ3/2 Ξπ

Combine Λ with π to reconstruct Ξ

Ξ3/2 Ξπ




Ξ3/2 Ξπ

No peaks observed in any of the combinations.

Ξ3/2 Ξπ

  • No Ξ3/2 state was observed.

  • The number of Ξ(1530) observed in this study is about the same as for the NA49 study.

  • The ratio R=N(Ξ3/2 )/N(Ξ(1530) ) around 1860 MeV is <0.29 at 95% C.L.

  • Since this is a search in the fragmentation region—does not necessarily contradict the NA 49 result.

C d p
ΘC  D*p

  • H1 collaboration has reported an observation of a narrow state ΘC  D*p at 3099 MeV in DIS.

  • The signal corresponds to ~1% of the observed number of D*s (51±11 ΘCs observed)

  • H1 also reports a compatible state observed in photoproduction (Q2≈0).

C d p1
ΘC  D*p

ZEUS has made a search in 126 pb-1 of DIS and photoproduction data.

Both K2π and K4π decay modes of D* were used. (H1 uses K2π only).

Altogether, 62600 D*s are found. 13500 in DIS alone. If 1% of D*s came from ΘC–expect >600 ΘCs.

C d p2
ΘC  D*p

dE/dx was again used to identify protons.

Procedure was somewhat refined—use χ2 probability of a proton hypothesis rather than a simple cut.

C d p3
ΘC  D*p

MC simulation of

ΘCat 1% of observed


“1%” signal is ruled out for the whole sample at 9σ

ruled out for DIS alone at 5σ

C d p4

K2π and K4π

-1.6 < η(D*) < 1.6

PT(D*) > 1.35 GeV (for K2π)

y < 0.95

K2π only

-1.5 < η(D*) < 1.0


0.05 > y > 0.7

PT(D*) > 1.5 GeV

For Photoprod.

0.2 > y > 0.8

PT(D*) > 2.0 GeV

..other minor changes

ΘC  D*p

Default selection

“H1” selection

Note all cuts are tighter

for the “H1” selection.

C d p5
ΘC  D*p

H1 selection

~5900 D*s

~11500 D*s

C d p6
ΘC  D*p

  • No resonance structure observed from >60k D*s.

  • “1%” signal is ruled out at 9σ for all and 5σ for DIS only.

  • (Also checked Q2>20 GeV2—nothing was found)

  • The upper limit of observed D*s originating from ΘCis 0.23% (95% C.L.)

  • After acceptance correction: (assuming PT(ΘC) and η(ΘC) same as those of D*): the upper limit of D*s, in the measured kinematic region, originating from ΘCis 0.16% (95% C.L.)

  • These results are incompatible with the H1 result.


  • Θ KSp : A narrow state observed at 1520 MeV at Q2>20 GeV2. Maybe the Θ+ pentaquark.

  • Ξ3/2 Ξπ : No state is observed. May still be compatible with NA49 result—fragmentation region probed.

  • ΘC  D*p : No state is observed. A production at 1% of the D* rate is ruled out. Not compatible with H1 result.